In many semiconductor device manufacturing processes, the required high levels of device performance, yield, and process repeatability can only be achieved if the temperature of a substrate (e.g., a semiconductor wafer) is strictly controlled during processing. To achieve that level of control, it is often necessary to measure the substrate temperature in real time and in situ, so that any unexpected temperature variations can be detected and corrected.
Such corrections may be accomplished by using process chambers with independent heating control over various portions of a substrate. For example, some process chambers include a plurality of heating elements, such as lamps, positioned over the substrate to be heated. Depending on the local temperature of the substrate, the power to these lamps may be varied to provide temperature uniformity across the entire substrate.
As an example of a fabrication process using such a plurality of lamps, consider rapid thermal processing (RTP), which is used for several different fabrication processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). In the formation of complementary metal-oxide-semiconductor (CMOS) gate dielectrics by RTO or RTN, film growth temperature is a critical parameter that influences device performance and fabrication yield. Currently, CMOS devices are being made with dielectric layers that are only 60-80 angstroms (.ANG.) thick and for which thickness uniformity must be held within a few percent. This level of uniformity requires that temperature variations across the substrate during high temperature processing cannot exceed a few degrees Celsius (.degree. C.).
The wafer itself often cannot tolerate even small temperature differentials during high temperature processing. If the temperature differential is allowed to rise above, for example, 1-2.degree. C./cm at 1200.degree. C., the resulting stress is likely to cause slip in the silicon crystal. The resulting slip planes will destroy any devices through which they pass. To achieve that level of temperature uniformity, reliable real-time, multi-point temperature measurements for closed-loop temperature control are often used.
One way in which the temperature is measured to achieve uniformity is optical pyrometry, which is widely used for measuring temperatures in RTP systems. Pyrometry exploits a general property of objects, namely, that objects emit radiation with a particular spectral content and intensity that is characteristic of their temperature. Thus, by measuring the emitted radiation, the object's temperature can be determined. A pyrometer measures the emitted radiation intensity and performs the appropriate conversion to obtain temperature. The relationship between spectral emitted intensity and temperature depends on the spectral emissivity of the substrate and the ideal blackbody radiation-temperature relationship, given by Planck's law: ##EQU1## where C.sub.1 and C.sub.2 are known constants, .lambda. is the radiation wavelength of interest, T is the substrate temperature measured in Kelvins (K), and I.sub.b (.lambda.T) is the spectral intensity as a function of wavelength and temperature. According to an approximation known as Wein's distribution law, this expression can be rewritten as follows: ##EQU2## where K(.lambda.)=2C.sub.1 /.lambda..sup.5. This is a good approximation for temperatures below about 2700.degree. C.
The spectral emissivity (.lambda.T) of an object is the ratio of its emitted spectral intensity I(.lambda.T) to that of a blackbody at the same temperature I.sub.b (.lambda.T). That is, ##EQU3## Since C.sub.1 and C.sub.2 are known constants, under ideal conditions, the temperature of the wafer can be accurately determined if (.lambda.T) is known.
However, despite its widespread use in the semiconductor industry, optical pyrometry still suffers from limitations due to its inability to accurately measure the emissivity of the substrate. Moreover, even if the emissivity of the substrate is known at a given temperature, it changes with temperature. The changes are usually not accurately measured. Thus, they can introduce an unknown error into the temperature measurements. Errors on the order of 10.degree. C. or more are not uncommon.
The emissivity of a substrate depends on many factors, including the characteristics of the wafer itself (e.g. temperature, surface roughness, doping level of various impurities, material composition and thickness of surface layers), the characteristics of the process chamber, and the process history of the wafer. Therefore, a priori estimation of substrate emissivity cannot provide a general purpose pyrometric temperature measurement capability.
Techniques for reducing the effect of changes in wafer emissivity on temperature measurements are known. One such technique involves placing a thermal reflector near the back surface of a target substrate to form a reflecting cavity which causes thermal radiation from the substrate to be reflected back to the substrate. A light pipe, which is inserted through the reflector into the cavity, samples radiation from the reflecting cavity and delivers the sampled light to a pyrometer. Assuming an ideal reflector, it can be shown mathematically that because all of the thermal radiation emitted from the substrate is reflected back onto the substrate, the reflecting cavity acts like an ideal black body. That is, the intensity of the thermal radiation within the reflecting cavity will not be a function of the emissivity of the surface of the substrate. Stated differently, in the ideal case, the reflecting cavity increases the effective emissivity of the substrate to a value equal to one. However, because the reflector will be less than perfect, the effective emissivity of the substrate will be higher than the emissivity of the wafer but less than one. Nevertheless, variations in the actual emissivity of the wafer will have considerably less impact on the measured temperature.
The above discussion relates to techniques for increasing the accuracy of substrate temperature measurements. These techniques use feedback to the heating sources to enhance substrate temperature uniformity.
Another way to increase substrate temperature uniformity is to use a temperature-sensitive process such as an oxide growth to grow a test film on a wafer. Oxide growth on silicon occurs at well-characterized rates for varying temperatures. By growing an oxide on silicon for a known amount of time and then measuring the thickness of the grown oxide as a function of the wafer radius using an ellipsometer or profilometer, the local temperature of the substrate may be obtained (also as a function of radius). Here, the term "local temperature of the substrate" is used to mean the temperature at a specified small area of the substrate, where "small" refers to a characteristic size over which the temperature variation is minimal.
The variation of thickness with radius may then be used as a guide to vary the power of the heat sources. For example, where the grown layer is too thick, the power to the heat source is lowered. This is referred to as adjusting the "offset" to a given zone of the heat source. An electrical offset is provided to the pyrometers (often just a value in .degree. C.) so that the pyrometer readings are adjusted in such a way so as to make the substrate temperature uniform.
However, this method often may not be ideal for growth systems such as epitaxial silicon deposition. One reason is that the gas chemistry in a growth chamber may be highly non-uniform because of gas flow dynamics as well as the cracking chemistry of epitaxial growth precursor gases such as SiHCl.sub.3. Such unstable gas flows may lead to uneven film growth which makes the measurement of the thickness of the grown film as a function of radius far less reliable.
Another reason why the above method may not be ideal for epitaxial systems is that, in the case of growing a test oxide film, it is sometimes seen that there is a chemical incompatibility between the process gases used for epitaxial growth, such as silane (SiH.sub.4), trichlorosilane (SiHCl.sub.3), etc., and oxygen sources.
Thus, though the above-mentioned schemes have achieved acceptable results, there is still considerable room for improvement, especially in the area of growth systems having multiple growth regimes such as epitaxial growth.